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Alkaline metals and their salts impart characteristic colour to an oxidising flame. This is because the heat from the flame excites the outermost orbital electron to higher energy level. When excited electron comes back to the ground state, there is emission of radiation in the visible region of the spectrum.

Beryllium and magnesium do not impart colour to flame because electrons in beryllium and magnesium metal are too strongly bound to get excited by flame.

What if reason that Be and Mg do not impart colour to flame is not that the electrons in Be and Mg are actually bound too strongly to be excited by flame but that the radiation emitted does not fall in visible spectrum?

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    $\begingroup$ Provide a reference to this text please. $\endgroup$
    – ACR
    Commented Jan 3, 2020 at 15:08
  • $\begingroup$ See: chemistry.stackexchange.com/questions/50467/… $\endgroup$
    – Nilay Ghosh
    Commented Jan 3, 2020 at 16:10
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    $\begingroup$ It is disappointing when the original poster vanishes after posting a question and won't follow up those who are trying to help. $\endgroup$
    – ACR
    Commented Jan 4, 2020 at 3:58
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    $\begingroup$ @M.Farooq Sorry I didn't see the notifications coming. The source is NCERT class 11 book, Unit 10 - The s-block elements. $\endgroup$
    – Rew
    Commented Jan 5, 2020 at 16:55
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    $\begingroup$ @Karl yes apparently. $\endgroup$
    – Rew
    Commented Jan 5, 2020 at 16:56

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You are right and whoever wrote this quote "Alkaline metals and their salts impart characteristic colour to an oxidising flame. This is because the heat from the flame excites the outermost orbital electron to higher energy level. When excited electron comes back to the ground state, there is emission of radiation in the visible region of the spectrum. Beryllium and magnesium do not impart colour to flame because electrons in beryllium and magnesium metal are too strongly bound to get excited by flame." is mostly wrong about Mg and Be.

The first process in flame spectroscopy is to decompose the compounds in the flame into atoms. /This requires a lot of energy. In ordinary lab flames, like that of the Bunsen burner (methane+air) the temperature is so low that alkaline earth (Ca, Ba, Sr) emit light because of their compounds in the flame not by their atoms. Recall magnesium and beryllium forms refractory oxides, so the correct explanation is that the flame temperature of ordinary oxidizing Bunsen burner is so low that it is not able to break magnesium compounds and excite beryllium in the flame.

Now if you use a proper flame, say that of acetylene-air for Mg or better acetylene-nitrous oxide flame for Be, you should see emission mostly in the UV but also very low intensity visible lines. The visible lines could be detected by a sensitive detector rather than the eyes. Nitrous oxide flame is highly colored itself (sunset pink color) and one cannot see visible emission easily without using a monochromator.

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  • $\begingroup$ And apparently, the text comes from India's National Council for Educational Research and Training! So ironic! Thanks for clearing me up on that one, really helpful. And sorry for I could not receive the notifications earlier. $\endgroup$
    – Rew
    Commented Jan 5, 2020 at 17:00
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    $\begingroup$ @Rew, Sorry to say but India's National Council for Educational Research and Training has to update itself. They have many outdated things. I see questions on chemical equivalents and n-factors here on these forums which were taught in the 1940s. $\endgroup$
    – ACR
    Commented Jan 5, 2020 at 17:29
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    $\begingroup$ Yes I agree. As an ex-user of NCERT, I have regretted studying from it and all I can say is that the textbooks are full of errors and misleading information which has to updated. $\endgroup$
    – Nilay Ghosh
    Commented Jan 5, 2020 at 17:53
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    $\begingroup$ @Nilay Ghosh, being outdated in syllabus was probably forgivable when internet did not exist. Today NCERT should see what is being taught around the world and compare the course contents and update them. There is a saying which is roughly translated as "If we lay the first brick of wall incorrectly, and if that wall reaches the stars, it will remain tilted." Therefore science at the school level has to improve so that students take interest in science from the beginning and better scientists can be produced at the university level. $\endgroup$
    – ACR
    Commented Jan 5, 2020 at 21:11
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    $\begingroup$ @AdilMohammed, Yes you are right. If enough heat were provided one would see atomic emission not molecular compound emission of low temperature flames. $\endgroup$
    – ACR
    Commented Mar 12, 2021 at 13:59

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